The applicability of the Volhard titration to the determination of small amounts of chloride~ in solid-rocket oxidants (ammonium nitrate, ammonium perchlorate, and potassium perchlorate) has not been ascertained prior to this time. In the course of determining the optimum experimental parameters of the Volhard titration, the recognition and empirical evaluation ot the operator determinate end-point error was required. The determination and use of this value, or use of a procedure which eliminates the need of its evaluation, is required when one titrates in solutions of high ionic strength, and is applicable to all analyses involving back-titrations and visual end-point detection.
The original Volhard 2 titration of silver has been utilized for the determination of chloride in acid solution. An excess of silver ion is added (I), and the excess is titrated with thiocyanate ion (2), in the presence of ferric ions, which serve as an internal Ag+ + C1--+ AgCl(s) (I)
indicator through formation of the familiar dark red coloration of the iron(III) monothiocyanate ion. In order to prevent metathesis of the silver chloride to silver thiocyanate (3) which occurs in the vicinity of the end-pointa, 4, nitrobenzene is added
as coagulating agent 5. After the equivalence point has been reached, a slight excess of thiocyanate ion is required to develop sufficient ferric thiocyanate ion to permit visual perception (4) : Fe+~ + CNS--+ Fe CNS +2 (4) * This paper presents the results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under Contract No. NASw-6, sponsored by the National Aeronautics and Space Administration.